While the following discussion will generally focus on purging, flushing and cleaning spray apparatus, it should readily be appreciated that the scope of the present invention is not limited to such apparatus. Indeed, its scope includes all apparatus in which it is capable to have a closed system so as to accommodate the compressed fluids which are utilized in the present invention.
Many materials are sprayed by a spray apparatus for different purposes, such as to apply the material to a surface, to foam the material, to disperse the material in droplet form into a gaseous carrier, to convert the material into particulate form, or to fabricate structural or composite materials. Materials that are spray applied onto a surface include coatings, adhesives, mold release agents, lubricants, detergents, insulation, herbicides, and the like. Materials that are spray foamed include flexible and rigid foamed plastics, foam rubber, foam insulation, and the like. Materials that are spray dispersed into a gaseous carrier such as air include fuels, pesticides, aerosols, and the like. Materials that are sprayed into particulate form include plastic microspheres, microballoons, spray-dried materials, and the like. Materials that are spray fabricated include structural plastics, reinforced plastics, filled composites, laminates, circuit boards, moldings, acoustical materials, carpet backing, coverings, insulation, and the like.
Extrusion in a fundamental sense is a similar operation to spraying in that material is passed through an orifice under pressure in order to apply it to something or to change its form for some purpose. The main difference is that the material is cohesive enough that it remains intact after passing through the orifice instead of subdividing into a spray of droplets. However, extrusion apparatus and spray apparatus are very similar in that they both provide pressurized material to the orifice and the material must be rendered fluid enough to pass through the orifice. Both types of apparatus utilize material supply systems, pumps, metering devices, flow control devices, heaters, tubing, and the like, and a spraying or extrusion device. Both operate under pressure and must be built accordingly. Both often use reactive materials. Both must be cleaned periodically to maintain proper operation. Often the same apparatus can be used for both spraying and extrusion by merely changing the material and the application orifice. Accordingly, in the present invention, the terms spraying, sprayed, and spray apparatus are understood to encompass within its scope, extrusion, extruded, and extrusion apparatus, respectively.
Many materials are extruded by an extrusion apparatus for different purposes, such as to apply a material to a surface, to form fibers or films, to fabricate structural or composite materials, or to fill voids such as a mold. Materials that are extrusion applied to a surface include sealants, caulks, adhesives, and viscous lubricants and greases. Materials that are extruded into fiber or film form include polymers used to make synthetic solid and hollow polymeric fibers, membranes, and plastic and photographic films, and the like. Materials that are extrusion fabricated include structural plastics, reinforced plastics, filled composites, laminates, moldings, coverings, insulation, and the like. Materials that are extruded to fill a mold include polymers used in injection molding, blow molding, and the like.
These materials often contain a solids or polymeric or hydrocarbonous fraction that is dissolved or dispersed in an organic solvent in order to liquify it or reduce its viscosity so that it can be sprayed. Or the material is heated in order to melt it or reduce its viscosity so that it can be sprayed. These materials often revert to their solid or highly viscous state as the organic solvent evaporates or the materials cool. Often the materials are reactive so that they solidify of their viscosity increases over time. Often different materials used in the same spray or extrusion apparatus are incompatible even at very low levels and contaminate each other if mixed. So too, the materials are often hazardous such as being toxic or flammable or unstable and therefore must not be left in the apparatus when it is idle, for safety reasons. For these and other reasons, the spray apparatus must be cleaned, that is, the material removed from the apparatus by dissolving or dispersing it in a cleaning solution, whenever the apparatus changes from spraying one material to another material or whenever spraying is finished or the apparatus is shut down or idled.
It is often also necessary to effectively remove conventional cleaning solution from a spray apparatus once cleaning has been effected, because the cleaning solution itself is incompatible with the next material to be sprayed or it is likewise hazardous. Contamination with cleaning solution may cause poor spray performance or poor product quality. Therefore, it is often desirable to flush the cleaning solution from the apparatus so that the apparatus is dry and free of cleaning solution before it is refilled with material to be sprayed.
To clean materials from spray apparatus, cleaning solutions consisting of one or more organic solvents are generally employed, due to their ability to dissolve organic materials. However, this leads to undesirable emission of organic solvent vapors into the atmosphere from such operations as filling and draining the apparatus, which saturates the air leaving and entering the apparatus with solvent vapor. Solvent vapors are emitted directly into the atmosphere when the solvent is sprayed from spray guns as they are flushed. In addition, air is often pulsed under pressure into the cleaning solution as it is fed into the apparatus in order to enhance cleaning action by increasing flow agitation and turbulence. These emissions cause air pollution and can be hazardous, because they expose workers to the vapors, and the cleaning solution is usually much more flammable than the material being sprayed. It is also much more susceptible to ignition by static discharge as it drains from the apparatus. Furthermore, the spent or used cleaning solution creates a large volume of hazardous waste material that must be transported and disposed of safely, which can be expensive.
Any cleaning solution left in the apparatus becomes mixed with the next material to be sprayed, so it is also emitted to the atmosphere. But drying the apparatus requires a large volume of air and often the air or apparatus is heated to increase the solvent vapor pressure, so more solvent is volatilized and emitted to the atmosphere and the vapors are more flammable.
In the liquid spray application of coatings, it is particularly true that the spray apparatus must be cleaned periodically during normal operation to prevent contamination or to prevent the coating material from setting up when the equipment is idle. The liquid spray application of coatings is effected mainly through the use of organic solvents as viscosity reduction diluents. However, as taught in the aforementioned related patent applications, supercritical fluids, such as supercritical carbon dioxide and supercritical nitrous oxide, have recently been found to be useful viscosity reducing diluents for the liquid spray application of viscous coating formulations, such as organic solvent-borne coatings and non-aqueous dispersion coatings, thereby markedly reducing the volume of environmentally undesirable organic diluents used for application. Supercritical fluids have similarly been found to be useful viscosity reducing diluents for the spray application of adhesives and also mold release agents such as waxes, oils, and greases. Supercritical fluids have also been found to have further utility as agents for creating feathered airless sprays and for creating wider airless sprays when spraying a variety of materials that also includes agricultural coatings, such as fertilizers and herbicides; chemical agents; lubricants; protective oils; non-aqueous detergents; and the like.
The aforementioned related applications also teach the addition of water to an active organic solvent-borne coating or adhesive composition such that when admixed with supercritical fluids, the water acts as an additional viscosity reduction diluent, which provides a composition having an even lower viscosity. This is surprising in that materials such as liquid or supercritical carbon dioxide are only sparingly miscible with water or water-borne polymer mixtures. In general, up to about 30 percent by weight of water, based on the total weight of solvent/diluent present, may be added without substantially reducing the amount of supercritical fluid that can be contained in the composition.
Usually, an organic coupling solvent is added in conjunction with the water addition, wherein it may indeed replace some of the nominal active organic solvent of the original composition, thereby maintaining the total amount of organic solvents in the water-containing composition at less than or equal to the amount contained in the original composition. Moreover, in some cases, even the total amount of volatile organic solvents needed can be reduced. The primary function of the coupling solvent is to enable a state to exist wherein all of the components in the composition mixture, namely the polymeric components, the water, and the active organic solvent (other than insoluble components such as pigments and the like) are in a single phase by virtue of its effect of creating miscibility or at least partial miscibility with one another of the components. The coupling solvent is a solvent in which the polymeric compounds used in the solids fraction is at least partially soluble, and is also at least partially miscible with water, thereby enabling miscibility of the solids fraction, the solvent fraction, and the water to the extent that a single phase is desirably maintained such that the composition may optimally be sprayed and a good coating or adhesive formed. The active organic solvents include those solvents which have particularly good solubility for the polymeric compounds that are used in the composition in addition to having at least partial miscibility with supercritical fluids.
Regardless of what liquid spray application is being practiced, even when using the improved technology, periodic cleaning is still essential for proper operation of the apparatus and for proper application of the coating. The problem is particularly acute in applications where colors or coating formulations are changed frequently, such as in industrial operations where articles or ware are to be spray coated at a spray station along a production or assembly line. When such operations require coating with a variety of colors, it is not generally realistic to have separate spray stations or production lines for each color, or even to spray a long line with one color and then change to another color, that is, to block operate by color. Therefore, it is more ideal to be able to make rapid color changes at a single spray station.
In many conventional systems, each color has its own supply container, feed pump, and feed system connected to a color control manifold, which is connected directly to the spray gun or other spray device. That is, each color has a redundant supply and feed system in parallel. Conventional process control devices, such as manual or automatic control valves, are operated manually or by a programmed automatic controller to give the proper sequence of colors for spraying and of cleaning solution and air for cleaning and purging the manifold and spray device. Although this type of color change system can readily spray a plurality of colors with a single spray device, there are economic disadvantages, mainly due to the large number of expensive pumps required. Another disadvantage arises from the time required to flush the manifold, which can become significant with high-solids coatings.
Many improvements are known to those skilled in the art, such as discussed, for example, in U.S. Pat. No. 4,337,282, issued Jun. 29, 1982, wherein the improvement embodies use of only two pumps, each connected to a color control manifold such that the pumps alternately supply different colors to the spray device, so that when one of the pumps is supplying material, the other is being cleaned. Nevertheless, much costly solvent is still needed and much hazardous waste is produced.
Another instance of a spray apparatus that experiences frequent color changes is automatic coating equipment on an automobile paint line, where color changes occur ordinarily from one automobile to the next as each automobile passes through the spray booth on a conveyor line. As discussed in U.S. Pat. No. 4,403,736, issued Sep. 13, 1983, a common technique is to use solvent at a relatively low superatmospheric pressure to flush the last of a quantity of coating of a given color from the spray apparatus and spraying device. The dilemma presented with this technique is that because of the relatively large capacity of the equipment, a large amount of spent solvent is expelled from the apparatus with high pressure air between each color change and is subsequently discarded. With several hundred such color changes per day per line in an automobile plant, a tremendous amount of hazardous waste is produced that contains organic solvents that are expensive, highly volatile, flammable, and toxic, so safety and environmental considerations are a concern, both inside the spray booth and in the air expelled to the environment. Furthermore, the large amount of hazardous waste generated by using said organic solvents must be contained, processed, and disposed of in an environmentally safe manner, which is also costly. It is apparent from the foregoing discussion that a reduction in the quantity of organic solvents used in said paint cleaning systems would be a significant benefit from economic, safety, environmental, and waste disposal considerations.
Improvements taught in the above-noted '736 patent include the following sequence at preprogrammed intervals: coating is supplied at a pressure of about 20 psia for a period of about 35 seconds; near the end of this period a valve is activated to introduce a "slug" of air at a slightly higher pressure for a few seconds to push the end of the first color from the manifold through the feed and spraying device; when the part being sprayed is past the spray apparatus, valves are activated supplying together a solvent and high-pressure air flush (at about 60 psig); finally, low pressure air is supplied to provide the next coating color.
Under certain conditions, however, the soft air push technique can experience difficulties. One can occur when electrostatic spray devices are used, which is quite common in the industry. As the coating material is pushed by air from inside the delivery tube, the coating material can break up and leave small pools inside the tube and on its wall. Because a different electrical potential can exist between these pools and the wall, arcing can occur. This can be hazardous because the coating and solvent vapors mix with the air and become combustible. Replacing the soft air push with a solvent push is taught as one solution to this problem. However, this increases the amount of solvent used, in turn leading to another embodiment, wherein a solvent system with vacuum capability is utilized to return solvent residing in the solvent delivery system to the supply tank, thereby conserving solvent, and in such a manner reducing the amount of solvent disposal required. Notwithstanding this approach, there still is a considerable amount of solvent being used in the solvent push and the solvent-high-pressure-air flush. It is clearly seen, if not for the hazard involved, the soft air push would undoubtedly be preferable because it minimizes solvent usage, costs, and the environmental impact.
In many industries there is a trend away from solvent based materials such as paints, lacquers, adhesives, and the like, in an effort to eliminate or reduce the amount of solvents discharged to the environment. In many instances these industries have gone to two component systems in which individual components are maintained separate through a metering process and only mixed and reacted just before application. As discussed in U.S. Pat. No. 4,265,858, issued May 5, 1981, these systems have suffered, however, from a lack of commercially available equipment for its application, particularly where several colors or materials are interchangeably applied through the same equipment. This patent calls attention to the fact that some provision must be made for completely purging the common equipment between the interchange of colors or materials. It further states that this purge must be made, for obvious reasons, with minimum loss of paint and time before this means becomes commercially acceptable. The lack of such quick color change apparatus has lessened the acceptance of this technology. The patent describes means and apparatus that are directed to solving this dilemma. In the methods and apparatus disclosed, the simultaneous matering and delivery of several metered flowable materials, in predetermined portions to a mixing device, is provided by animproved modular control system for promoting rapid interchanging of the flowable material. Wherein, when it is desirable to change colors, a control valve is actuated to cause solvent to flow through the common elements of the system to purge these elements of the old material after its flow is shut off. The solvent is delivered to a solvent aspirator wherein high pressure air is also introduced. The solvent-air mixture then flows through the common passages of the paint manifold, the mixer, and to the gun. In most instances after the gun is purged, the flush bypasses said gun exiting the system through a dump valve positioned adjacent to the gun. After the solvent purge, the new differently colored material is caused to flow into the manifold. Here again large amounts of purge solvent must be handled with all of the aforementioned disadvantages.
Other means and apparatus have been developed to operate in the liquid urethane paint industry wherein two component systems, one a color component and the other a catalyst component, are relatively stable until combined just prior to application. One of the consequences of this highly reactive mixture is its short pot life, thereby requiring purging of this part of the system of residue every few minutes; otherwise, the system will eventually become inoperable. This requires that special handling, mixing, and solvent purging apparatus be developed especially for this type of paint. Such a system is discussed in U.S. Pat. No. 4,019,653, issued Apr. 26, 1977. This system, as with the others previously described, also suffers all of the disadvantages heretofore characterized regarding solvent purging and flushing.
Organic solvents are normally used to clean coating materials from spray apparatus by dissolving, diluting, and displacing it. The organic solvents usually have a lower flash point and are more flammable than the coating materials themselves. Therefore, as already disclosed, the cleaning operation may entail a greater hazard than the coating operation. Furthermore, as already seen, the use of organic solvents for cleaning and flushing spray apparatus can cause the emission of volatile solvents to the atmosphere. In many instances, for example, as earlier described with color changes, air is pulsed into the solvent to promote turbulence in the flow to more effectively remove the coating formulation. In which case, this air is vented to the atmosphere saturated with organic solvent. As already seen, the cleaning operation with organic solvents also creates a large amount of hazardous waste to be disposed of safely.
The general use of compressed fluids as cleaning vehicles are known to those skilled in the art. For example, Whitlock in U.S. Pat. No. 4,806,171, issued Feb. 21, 1989, discloses an apparatus for removing submicron particles from a substrate by projecting a stream containing solid (dry ice snow) and gaseous carbon dioxide, which is produced by expanding liquid carbon dioxide through several chambers and an exit port, toward the substrate whereupon said stream blows across the surface, removing the particles without scratching the substrate. Berg in U.S. Pat. No. 3,947,567, issued Mar. 30, 1976, discloses cleaning compositions exhibiting effervescence, which comprise a cleaning agent and a liquefied gas present in the compositions at pressure and temperatures at which it would normally exist only in the gaseous state, wherein as vapors of the liquefied gas separate from the compositions, effervescence occurs. These cleaning compositions are maintained in an aerosol container and include such cleaning compositions as mouthwash, breath freshener, toothpaste, soaps, shampoos, drain cleaners, sink cleaners, rug cleaners, and the like. A variety of liquified gases are suitable. Typical of such materials are octafluorocyclobutane, chlorodifluoromethane, propane, butane, cyclobutane, pentane, and mixtures thereof. The compositions can also contain dissolved gases together with the liquefied gases such as carbon dioxide, nitrous oxide, and air. The liquefied gases used are those with boiling points ranging from about -50 F. to about 80 F with vapor pressures, at the upper temperature level, typically of around 150 psig.
Another common approach is cleaning by employing supercritical gas in a pressure vessel. An example is Japanese Patent No. 59,502,137, dated Dec. 27, 1984, where contaminants produced during the manufacture and processing of a solid structural component or element are removed from its surface by contact with supercritical gas in a pressure vessel. Preferably the treatment is effected with carbon dioxide at a temperature of 35 C. to 100 C. and a pressure of 1500 to 10000 psi. The contact time may be 0.25 to 4 hours. Material surfaces that may be cleaned include metals, rubber, synthetic polymers, carbon and quartz crystals. Another example is described in Japanese Patent No. 61,177,301, dated Aug. 9, 1986, in which a mixture of heat resistant material powder and binder is preformed and all preformed surfaces are coated with a substance to be removed by heating and lowering the pressure. The preformed coating is then pressure formed in a vessel by high isostatic pressure and successively contacted with supercritical fluids. The heat resistant material may be metal, metal oxide, ceramics, etc. The binder and coating substance may use higher alcohol, fatty acid polyethylene, etc., and the supercritical fluid may be carbon dioxide or fluorohydrocarbon. The isostatic pressure ranges from 1422 to 14223 psi.
The semiconductor industry also utilizes supercritical fluid, especially carbon dioxide, for cleaning. Japanese Patent No. 01,045,131, dated Feb. 17, 1989, teaches the washing and oxidizing of a semiconductive wafer by washing it first with supercritical or liquefied carbon dioxide and then the wafer is contacted with carbon dioxide including at least one kind of substance having oxygen to oxidize the silicon surface of the semiconductive wafer. The washing and oxidizing is performed in one tank, thereby reducing the possible contamination of the wafer by exposure to the atmosphere. Japanese Patent No. 60,192,333, issued Sep. 30, 1985, discloses a method for removing a hardened organic film from the substrate to which it is bonded, and in particular concerns a method which is suitable for mechanically peeling off the coated film of a photoresist coated film on a semiconductor wafer. In this case, the substrate with the hardened bonded organic film is first put under high pressure, mixed with a liquified gas and then brought into contact with a supercritical gas, after which the temperature and pressure conditions are changed to cause the gas to expand, and the hardened organic film is removed from the substrate by this expansion force. The liquid gas or the supercritical gas is dissolved either on the hardened organic film itself or at the interface between the organic film and the substrate. When either the pressure is decreased and/or the temperature is increased, the dissolved gas inside the hardened organic film or in the interface between the film and the substrate expands, resulting in the exfoliation of the hardened organic film from the substrate. The preferred liquified gas or supercritical gas is carbon dioxide, in which case it is desirable to add an organic solvent in which carbon dioxide is highly soluble to improve permeation into the hardened organic film and its substrate.
Another method is disclosed in U.S. Pat. No. 4,238,244, issued Dec. 9, 1980, which uses the technique of raising and lowering pressure to produce gas bubbles for removing inorganic deposits from industrial equipment such as heat exchangers. It is different from the above discussed methods in that it generally circulates the cleaning liquid through the apparatus. The method disclosed is specifically the removal of deposits of corrosion products and scale from the interior surfaces of heat transfer equipment with a liquid composition capable of removing said deposits under appropriate contact conditions of pH, temperature, concentration, and pressure for a period of time sufficient to remove the deposits. The deposits cited to be removed include inorganic materials such as metal oxides, spinels, metal sulfides, and water scale such as gypsum and magnesium oxides and others. The liquid cleaning compositions cited for removing the deposits include inorganic and organic acids, salts of such acids, and inorganic and organic bases. The method generally calls for dissolving at super atmospheric pressure a chemical that is a gas at atmospheric conditions to form a solution that produces a gas when at reduced pressure. The preferred gas is carbon dioxide. The procedure comprises: contacting the deposits with the solution for an initial period of time; contacting the deposits with this solution for an additional period of time at a reduced pressure, wherein carbon dioxide is liberated from the solution and said solution is therein agitated during such said contacting; and repeatedly raising and lowering the pressure exerted on the solution while contacting the deposits such that carbon dioxide is repeatedly placed in solution and liberated therefrom, thereby causing agitation which improves the deposit removal. The super atmospheric pressure mentioned ranges from above atmospheric up to about 1500 psig at temperatures in the range of from atmospheric to about 350 F. (about 177 C.). While it is indicated that various concentrations of the gas-forming substance can be used, it is asserted that concentrations in the range from about 0.1 percent to about 5 percent by weight of deposit-removing liquid has been found to be effective. The deposit-removing liquid that consists of acids, bases, or salts constitute a significant majority of the combined deposit-removing, liquid-gas-forming admixture. Whereas such a method enhances the removal of inorganic deposits and scale, it does not significantly result in a reduction in the use of the deposit removal cleaning solution. In addition, this method practices the fluctuation of temperature and pressure condition repetitiously to provide the improvement cited, which is related to formation of gas bubbles which provide the agitating force. Practicing said method would generally suggest the necessity of utilizing an intricate monitoring, control, and operating process for the method to be effective and practical. Such devices are costly and would add to the overall capital, period, and operating costs. Furthermore, the highly corrosive cleaning solution that consists of acids, bases, or salts employed for removing the inorganic deposits is inappropriate and incompatible with removing organic deposits such as coating formulation and polymers from spray apparatus.
In the methods of the present invention, wherein spray apparatus is purged and cleaned between coating material change or at shut down, organic solvents constitute not only part of the coating formulation but the cleaning solution as well. Since these organic solvents generally are potential pollutants, their containment and waste disposal dictate minimal usage. Such would not be the case if the method discussed in the aforementioned patent was practiced within the constraint implied by the very low range of concentration of the gas-forming substance in the deposit-removing liquid. Moreover, with coating formulations admixed with supercritical fluids such as carbon dioxide as diluents, lowering the pressure to levels below the critical point could result in the formation of two phases plus significant vaporization of carbon dioxide, wherein pockets high in carbon dioxide concentration may form and contact the coating admixture, from which could result undesirable deposition of coating materials on conduit walls and on internal surfaces of the apparatus, which causes deposits that are harder to remove. In fact, under the worse of conditions, highly viscous pure, or nearly pure, polymer could come out of solution, indeed presenting a most difficult and costly removal condition.
Clearly, what is needed is a means for purging, flushing and cleaning apparatus, particularly spray apparatus between color or material changes, and at equipment shutdown that has low organic solvent usage, low cost, low hazard, and minimal environmental impact and that minimizes the creation of hazardous waste. A similar means is needed for purging cleaning solutions from the spray apparatus following the cleaning operation.
Such a means has now been found by the use of supercritical fluids, such as supercritical carbon dioxide or nitrous oxide that are utilized in their supercritical or near-supercritical fluid state which have been discovered to have favorable utility in purging, flushing and cleaning spray apparatus and have also been found to have the above mentioned advantages. They have been further discovered to have favorable utility in purging cleaning solutions from spray apparatus following cleaning.